There are various external methods of fixing a fractured bone, such as the radial external fixation system, the femoral standard frame and the tibial standard lengthening frame which are made by the Daruma Corporation of the United States. Such conventional methods as mentioned above are defective in design in that they are complicated in construction and that they hamper the mobility of a patient under treatment. For this reason, the orthopedic surgeons are more inclined to make use of monotube fixation devices, such as the articulated components introduced by EBI Medical Systems, Inc. of the United States, and the external fixation system made by the Howmedica Corporation of the United States and sold under the trademark Mono-tube .TM.. These monotube fixation systems involve the use of a clamping structure to hold firmly the bone screws which are fastened onto the bone. As shown in FIG. 1a, the clamping holes A, b and C are aligned while the screws AA', bE' and CC' are so.oriented that they are in the same plane designated by ACC'A' and that they are parallel to one another. As a result, the fixing effect on the bone of such monotube fixation systems as described above is generally unsatisfactory, to say the least.